I'm losing it
Someone's running out of patience. When a situation is even too much stressful.
Too many streaming children here. I'm losing it.
I'm about to lose my marbles
Being in the the state of confusion and absolutely going bonkers. The person who says this phrase usually states it in a tone where they are rather stern and will get ready to square up on the spot (known from experience).
Shannon- Sweetie, are you alright? You're strangling the dog again!
Bertha- I'm about to lose my marbles!
Bertha- I'm about to lose my marbles!